1. Field of the Invention
The present invention relates to accelerometers including a silicon capacitive acceleration detector, and to a method for measuring the capacitive unbalance of the detector, providing an electric signal representative of an acceleration.
2. Discussion of the Related Art
A schematic exemplary silicon capacitive acceleration detector is illustrated in the cross-sectional view of FIG. 1. Detector 1 includes a central silicon plate 2 sandwiched between two external silicon plates 3 and 4. The central silicon plate 2 is etched prior to being assembled so as to include a frame 5 and a central plate, or pendulous mass 6, that is fastened to frame 5 by one or more holding arms 7. The insulation of the frame 5 of the central plate from the external plates 3 and 4 is provided by insulating strips 8 and 9, generally of silicon oxide. The external plates 3 and 4 delineate with frame 5 a space within which is suspended the pendulous mass 6. The upper and lower surfaces of the pendulous mass 6, as well as the surfaces of plates 3 and 4 facing the pendulous mass 6, include conductive surface areas, or electrodes 10, 11, 12, 13, forming a system including two capacitors Cs, Ci that are symmetrically disposed with respect to the median plane of detector 1. Electrodes 10-13 are accessed through contact pads and internal connections (not shown in FIG. 1). Usually, the mobile electrodes 10 and 11 are not insulated from the pendulous mass 6 (they are then doped silicon areas) and are at the same potential; so, a single contact pad is provided for electrodes 10 and 11.
When the device is at rest, capacitors Cs and Ci have substantially an equal value, as follows: EQU cs=ci=.epsilon.S/do
where .epsilon. is the dielectric constant of the gas present in detector 1, S is the surface area of electrodes 10-13, and do is the distance separating, at rest, the pendulous mass 6 from each external silicon plate 3 and 4.
When the device withstands an acceleration, the pendulus mass 6 moves, with respect to its null position, by a quantity z proportional to the acceleration. In this case, capacitors Cs and Ci vary and have the following values: EQU Cs=.epsilon.S/(do+z) (1) EQU Ci=.epsilon.S/(do-z) (2)
where z is an algebraic length, whose sign is conventionally determined.
To measure acceleration, a system for measuring the unbalance of capacitors Cs and Ci is combined with the above described detector.
FIG. 2 represents the electric diagram of a conventional accelerometer 20 including the above-described detector 1 and such a measurement system. Detector 1 is represented by the two capacitors Cs and Ci that are formed by electrodes 10, 12 and 11, 13; the pad common to electrodes 10 and 11 being represented by a node 14. The measurement system includes an excitation unit 21 for exciting capacitors Cs and Ci, that is fed by a reference a.c. voltage v, a unit 30 for processing the signal from the detector, providing a measuring voltage vs, and a feedback loop 26 connecting voltage vs at the input of the excitation unit
The excitation unit 21 drives the fixed electrode of capacitor Cs through a differential amplifier 22, and the fixed electrode 3 of capacitor Ci through a summing amplifier 23. The negative input of amplifier 22 and a first input of amplifier 23 are connected to an amplifier 24 having a gain b, receiving the reference voltage v. The positive input of amplifier 22 and the second input of amplifier 23 are connected to the output of an amplifier 25 having a gain a, receiving the voltage vs provided by the processing unit 30 through the feedback loop 26.
The processing unit 30 mainly includes a current/voltage converter 3 having its input connected to node 14, followed by an amplifier 33 having a very high gain G, providing the measuring voltage vs. In FIG. 2, converter 31 is schematically represented by an operational amplifier 32 having its output connected to its input through a capacitor having a capacitance Cr.
Voltage vs is used as the output voltage of the measurement system. Voltage vs is rectified in a demodulator 40 that is connected to the output of the feedback loop 26 and synchronized with voltage v, and that provides a voltage Us constituting the output signal of the accelerometer.
The above description shows that the excitation unit 21 applies to the fixed electrodes 12 and 13 of capacitors Cs and Ci excitation voltages, avs-bv and avs+bv, respectively. The input of the current/voltage converter 31 collects a differential current from node 4 resulting from the excitation of capacitors Cs and Ci. At the output of the processing unit 30, voltage vs is: EQU vs=K(Cs-Ci)/(Cs+Ci), (3)
where K is a constant.
The theoretical advantage of such a measurement system is that the amplitude of voltage vs is proportional to displacement z of the pendulous mass 6 of the detector and, hence, to the acceleration. In combining equations (1), (2) and (3), it can be appreciated that vs=K z/do.
However, in this prior art accelerometer, the measurement of the displacements of the pendulous mass is actually significantly affected by the presence of high stray capacitances present in the detector, to such an extent that the output signal Us obtained by demodulation of voltage vs is erroneous. Thus, it is noted that the output signal does not linearly increase as acceleration increases, in contrast to what is expected from the above theoretical equation (3). Such stray capacitances (labeled as C1 and C2 in FIG. 2) are predominantly formed in the region of the insulation layers 8 and 9 between frame 5 of the central silicon plate 2 and the corresponding surfaces of the external plates 3 and 4.
To avoid this drawback, various technological approaches, aiming at modifying the detector structure in order to reduce stray capacitances, have been proposed. However, these technological approaches involve an increase in the cost and/or the complexity of the detector.
The applicant proposes a fully different approach, consisting in reducing the influence of the stray capacitances in the measurement system instead of modifying the structure of the conventional detectors.